1. Technical Field
Features described herein relate to an image forming apparatus for forming an image on a printing medium.
2. Related Art
A color ink-jet printer is known as an imaging forming apparatus. In this printer, ink droplets are ejected sequentially to a printing medium while an ink head for ejecting plural ink droplets is moved in the main scanning direction. After a scan in the main scanning direction has finished, an auxiliary scan is performed by, for example, moving the printing medium in a direction that crosses (e.g., is perpendicular to) the main scanning direction and then a main scan is performed again.
In the ink head, discharge apertures (printing elements) for ejecting plural ink droplets are arranged in the auxiliary-scanning direction. From a main scan, these apertures eject ink droplets onto a printing medium, forming rows of plural dots arranged in the main scanning direction (sometimes referred to as “rasters”) When the printing medium is thereafter moved in the auxiliary-scanning direction by a ink head length, there may occur a phenomenon that the interval between the tail raster formed by the preceding main scan and the head raster formed by the current main scan becomes wider than the interval between rasters that are formed by one main scan due to, for example, an error of a feed mechanism for feeding the printing medium. In this case, white streaks occur which are called banding.
FIG. 6A shows example banding that occurs in the above manner. An ink head 70 is formed with 100 discharge apertures in the auxiliary-scanning direction. Dots indicated by white circles are formed by a main scan (preceding main scan) by the 97th to 100th discharge apertures of the ink head 70.
Then, after the printing medium has been transported in the auxiliary-scanning direction, dots indicated by black circles are formed by the 1st to 4th discharge apertures of the ink head 70. If the printing medium is transported by 101×d by the auxiliary scan (d: is the pitch of the discharge apertures of the ink-jet head 70), the interval between the tail raster formed by the preceding main scan and the head raster formed by the current main scan has a normal value. However, assume that the printing medium has been transported excessively (excess distance: Δ). In this case, the distance between the raster formed by the 100th discharge aperture in the preceding main scan and the raster formed by the 1st discharge aperture in the current main scan is equal to d+Δ. The raster interval there is wider than the other intervals d by Δ, which means banding.
In one known method for reducing the degree of such banding, as shown in FIG. 6B, the printing area of a preceding main scan and that of a current main scan overlap each other, so that for the rasters in the overlap area, some of the dots are printed by the preceding main scan, and the remaining dots are printed by the current main scan.
In the example of FIG. 6B, the rasters of the overlap area are formed by the 99th and 100th discharge apertures of the ink head 70 in the preceding main scan and by the 1st and 2nd discharge apertures of the ink head 70 in the current main scan.
FIG. 6C shows a case of the FIG. 6B technique, in which the printing medium was transported excessively (excess distance: Δ) in the auxiliary-scanning direction. Dots formed by the 1st discharge aperture of the ink head 70 are deviated downstream by Δ from dots formed by the 99th discharge aperture of the ink head 70, and dots formed by the 2nd discharge aperture of the ink head 70 are deviated downstream by Δ from dots formed by the 100th discharge aperture of the ink head 70. However, since the gaps caused by these deviations are not located on straight lines, they are less noticeable, and the degree of banding can be reduced.
However, in the technique discussed above, one raster is printed by plural main scans in an overlap printing area. Therefore, if an error occurs between a preceding main scan and a current main scan, dots formed by the preceding main scan and dots formed by the current main scan may overlap with each other. Such overlapping dots may be more noticeable to the human eye, and is unacceptable in view of increasing demand for higher print accuracy. Improving print accuracy is also hampered by the increasing demand for higher print speeds. As the movement speed in the main scanning direction is increased, the shape of the dots formed becomes less of a circle and more like an ellipse that is long in the main scanning direction.
As the printing speed is increased, the time interval between dot formation by a preceding main scan and that by a current main scan (i.e., the time from landing of ink droplets onto a printing sheet in the preceding main scan to landing of ink droplets in the current main scan) becomes shorter, and the current ink droplets land before the preceding ink droplets dry.
Therefore, if a deviation occurs in the main scanning direction, dots formed by the preceding main scan and dots formed by the current main scan may overlap with each other. When a dot of the current main scan is superimposed on a dot of the preceding main scan before the latter dries, then the two dots are combined into a single dot having a larger diameter, resulting in a deterioration in dot graininess.
FIG. 6D shows how elliptical dots formed by a preceding main scan and elliptical dots formed by a current main scan are connected to each other. If dots are connected to each other in this manner, small dots are particularly deteriorated in graininess when combined into larger dots. This means a problem that a rough, grainy image is formed instead of an intended high-resolution image.